Method of producing nodular iron



Feb. 5, 1963 B. C. ,YEARLEY METHOD OF PRODUCING NODULAR IRON Filed Feb. 8, 1960 2 Sheets-Sheet 1 INVENTOR.

Ame y Feb. 5, 1963 METHOD OF PRODUCING NODULAR IRON Filed Feb. 8, 1960 2 Sheets-Sheet 2 INVENTOR.

3r Jr 2412 B. c. YEARLEY 3,076,705

United States Patent 3,076,795 METHUD (33F PRUDUCENG NODULAR IRQN Bernard C. Yearley, Shaker Heights, Ohio, assignor, by

mesne assignments, to Malieabie Research and Development Foundation, Dayton, fihio Fiicd Feb. 8, 1969, er. No. 7,166 8 Claims. (Cl. 75-130) This invention relates to the production of nodular I iron in the as-cast state, and more particularly to a process for producing nodular iron castings from any molten iron by treating the molten mix with a pretreatment slag that increases the responsiveness of the mix to a nodulizing agent such as magnesium.

In the literature of the art, the term nodular iron is used to indicate an iron in which the microstructure of the graphitic carbon present in the casting is in the form of nodules. The word nodular as employed in the art, and for purposes of the disclosure of this invention, is not restricted to a truly perfect spherulite, but includes any form of graphitic carbon other than lenticular or flake form. By altering the structural configuration of the graphitic carbon, one skilled in the art can alter the physical properties of the metal and thus expand the uses of nodular iron castings.

As is well known to those skilled in the art, the usual method of producing nodular iron in the as-cast state is to add to the molten mix, prior to pouring or casting, a nodularizing agent such as magnesium, cerium, or misch metal. However, due to the cost of suitable nodularizing agents, among other reasons, it is desirable to keep the quantity of nodularizing agent used as small as possible. Recent work in the art has been directed toward increasing the responsiveness of the molten mix to the nodularizing agent by treating the mix, prior to inoculation with a noduiarizing agent, with a special slag, known as a pretreatment slag. Most of these efforts have resulted in limitations in the chemical make-up of the molten mix that can be used, either due to the particular nodularizing agent employed, or to the type of furnace that can be used in reducing the charge.

One form of limitation has been a restriction in the type of cupola in which the charge is reduced. A basic lined furnace can only use a basic pretreatment slag. An acid cupola, likewise, is restricted to an acidic pretreatment slag. A neutral lined furnace, of course, can use either slag. Another limitation, although frequently not readily apparent, is that many of these processes require the use of what is known as a premium charge of pig iron. All of these factors afiect the quantity of iron that can be produced within a given period of time, as well as the cost of production.

An ideal process for the production of nodular iron would be one with which it is possible to take any molten iron mix, raise the carbon content if necessary, treat with a pretreatment slag, then inoculate with a suitable nodularizing agent to produce the desired iron.

It is therefore the primary object of this invention to provide a process for the production of nodular iron in the as-cast state wherein there is little limitation on the chemical composition of the molten iron or of the nodularizing agent employed.

Another object of this invention is to provide a pretreatment slag for use in a molten iron mix to desulphurize the mix and increase its responsiveness to a nodularizing agent prior to casting.

A more specific object of this invention is to provide a pretreatment slag with a predetermined critical minimal composition for use in a molten iron mix to desulphurize the mix and increase its responsiveness to a nodularizing ,agent prior to casting.

Patented Feb. 5, 1963 Other objects and advantages of this invention will be apparent from the following description taken in conjunction with the drawings in which:

FIG. 1 is a reproduction of a photomicrograp'h taken at a magnification of 100 times of cast iron treated while in the molten state, with a pretreatment slag having 7.55

by weight of the slag of fluorspar;

FIG. 2 is a reproduction of a photornicrograph taken at a magnification of 100 times of cast iron treated, while still molten, with a pretreatment slag having 11.3% by weight of the slag of fluorspar;

FIG. 3 is a reproduction of a photomicrograph taken at a magnification of 100 times of cast iron treated, while still molten, with a pretreatment slag having 15.1% by weight of the slag of fiuorspar;

PEG. 4 is a reproduction of a photomicrograph taken at magnification of 10!) times of cast iron treated while still molten, with a pretreatment slag having 18.8% by weight of the slag of iiuorspar.

A cupola furnace with an acid lining, normally operating on a large production basis, will have in its charge a large quantity of sprue. The term sprue as used herein is used to indicate gates, runners, risers and rejected castings from previous heats. This sprue, coupled with the usual scrap iron charge, is prone to give the molten mix a high sulphur content. it has b%n found that for a nodularizing agent to function effectively, the sulphur content of the mix must be in the range of .006 to .02% by weight of the mix. It is also known in the art that many nodularizing agents can desulphurize a mix, but this isnct desirable for the reasons already stated. Hence, to obtain a molten mix that is amenable to a nodularizing agent, without using the nodularizing agent itself to desulphurize, the operator of'an acidic cupola must start with a premium charge high in pig iron so as to keep rigid control of the sulphur. Such a procedure is not only time consuming, but costly as well.

in addition to the problem of tne chemical make-up of the molten mix, there is presented the problem that an acid lined cupola cannot use a basic pretreatment slag without damage occurring to the cupola walls. Hitherto in the art, it has been the practice merely to add a pretreatment slag directly to the molten mix in the cupola. Such a practice has obvious limitations in application due to the limited availability of suitable pretreatment slags. There are no known acidic slags that will work consistently except when desulphurization is performed as a separate step, hence the operator of an acidic cupola cannot consistently produce large quantities of as-cast nodular iron.

To overcome these and other difficulties, the following process embodying the present invention can be used to make production quantities of as-cast nodular iron. The process first requires that a typical cupola charge, high or low in carbon, with its typically high sulphur content be reduced to a molten mix. 'At this point in the process it is immaterial whether the operator has a white iron or gray iron composition, or whether the sulphur content is .005% or .2 by weight of the molten mix. if the carbon content is too low to have sufiicient graphitic carbon present to nodularize, a suitable addition to the mix can be conveniently made at this time. Nor is it necessary to reduce the charge in an acidic lined or basic lined cupola, it is merely necessary that the furnace employed be suitable for melting the charge. After bringing the mix up to the desired carbon content, it that is necessary, the molten mix is transferred to a furnace in which reducing conditions can be produced and where the pretreatment slag is brought into contact with the molten mix. a

The pretreatment slag will reduce the sulphur content to a desirable level, as well as in some manner not completely understood, prepare the mix for the addition of a minimal amount of magnesium as the nodularizing agent. As a result of such pretreatment of the mix, the amount of magnesium added to the molten mix necessary to eflect complete nodularization has been found to be as low as .06% of the mix by weight, a figure hitherto thought impossible. While there is some flexibility in the chemical composition of the pretreatment slag, the preferred composition is as follows:

16-21% of the slag by weight of calcium fluoride (fluorspar) 310% of the slag by weight of carbon (coke) 69-81% of the slag by weight of calcium magnesium carbonate (dolomite) The constituent of prime importance in the slag is the halogen of calcium. The minimal amount of calcium fluoride that may be used has been established as 15.0% of the slag by weight. It has been found that good nodular iron cannot be consistently produced unless this minimum is observed. There does not appear to be any maximum to the amount of halide that may be used in the slag. There is every indication that as the amount of halide in the slag is increased, there can be a corresponding decrease in the other constituents of the slag.

Referring first to the FIG. 1 photomicrograph of cast iron, the pretreatment slag employed in the production of the iron contained only 7.55% by weight of calcium fluoride. As is readily apparent, few spherulites, or nodules, are present in the casting. The FIG. 2 photomicrograph of cast iron Was obtained when the pretreatment slag employed container 11.3% by weight of calcium fluoride. In this section the number of spherulites present has increased. However, there are still present large quantities of graphite in flake form which appears in a dendritic pattern.

FIG. 3 is a photomicrograph in which the pretreatment slag employed contained 15.1% of calcium fluoride by Weight. This amount of the fluoride of calcium, or its mol equivalent of the other halogens (bromine, chlorine and iodine), appears to be the minimal amount that can be present in the pretreatment slag and still produce nodular cast iron consistently. Throughout this disclosure, calcium fluoride has been employed as an example because of its use in the preferred embodiment of the pretreatment slag. However, many other metals appear to be freely interchangeable with the calcium, for it is the halogen portion of the molecule that enters into the chemical reaction that is significant in the production of nodular iron, and not the calcium.

FIG. 4 is a photomicrograph of cast iron in which the pretreatment slag employed in its production contained 18.8% of calcium fluoride by weight. A comparison between FIGS. 3 and 4 shows clearly there is no appreciable difference in the number of nodules present. This seems to indicate that there is little to be gained from using a pretreatment slagthat contains substantially more than about 20% by weight of calcium fluoride. Significantly, it has also been determined that any halide of calcium can be substituted. The chloride of calcium, although deliquescent, can be employed providing some care is used in preventing an excessive amount of moisture from becoming associated with the substance. The use of bromine and iodine, although possible, is not recommended for large scale commercialproduction of nodular iron. In this process the pretreatment slag should remain in contact with the molten mix for approximately 20 minutes at 2850" F. for maximum efficiency. A mix very high in sulphur will require a corresponding increase in the quantity of pretreatment slag used. For a mix having a sulphur content in the range of .12 to .18% by weight of the mix, the total quantity of pretreatment slag should be approximately by weight of the mix.

A mix with a sulphur content of approximately .06% of the mix by weight will require a slag that is approximately 5% by Weight of the mix. A corresponding increase in the amount of nodularizing agent employed would be required with the higher sulphur contents if the quantity of pretreatment slag remained constant.

It is to be understood that this process is not restricted to the use of any particular nodularizing agent, for it can be employed with almost any nodularizing agent, but the preferred agent is magnesium.

What has been disclosed, therefore, is a unique process for the production of nodular iron in the as-cast state that permits any molten iron mix to be used at the start with no limitations imposed on the type of furnace used in reducing the charge. By subjecting the molten iron mix to the disclosed pretreatment slag, then inoculating with a suitable nodularizing agent such as magnesium, one is able to make production quantities of satisfactory nodular iron with remarkable consistency.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features of the'process described, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. The method making was cast nodular iron comprising the steps of contacting the molten metal with a pretreatment slag comprising 15% to 21% by weight of a halogen of calcium, accompanied by 3% to 10% carbon, and the balance calcium magnesium carbonate in a basic or neutral vessel and then introducing magnesium as a nodularizing agent prior to pouring castings.

2. The method of making as cast nodular iron in the 2% to 4.2% carbon range comprising the steps of contacting the molten metal with a pretreatment slag comprising 16% to 21% by weight of a halide of calcium, aecompanied by 3% to 10% by weight of carbon, and the balance calcium magnesium carbonate and then introducing magnesium as a nodularizing agent prior to pouring castings.

3. The method of making nodular iron as set forth in claim 2 wherein the pretreatment slag comprises 15% to 21% by Weight of a halogen of calcium, accompanied by 3% to 10% by weight of carbon and the balance calcium magnesium carbonate, and then introducing a nodulanizing agent.

4. The method of making as cast nodular iron from a molten metal comprising 2% to 4.2% carbon by weight and 005% to .20% sulphur by weight comprising the steps-of cont-acting the molten metal with a pretreatment slag comprising 15% to 21% by weight of fluorspar, accompanied by 3% to 10% carbon, and the balance calcium magnesium carbonate, and then introducing magnesium as a nodularizing agent prior to pouring castings.

5. The method making as cast nodular iron comprising the steps of contacting the molten metal with a pretreatment slag comprising 15 to 21% by weight of a halogen of a calcium accompanied by 3% to 10% by weight of carbon, and the balance calcium magnesium carbonate, then introducing a nodularizing agent.

6. The method of making as cast nodular iron from a molten metal comprising 2% to 4.2% carbon by weight, and 005% to .20% sulphur by weight, comprising the steps of contacting the molten metal with a pretreatment slag comprising 2 parts fluorspar, 1 part coke, and 7 parts dolomite by weight, then inoculating the mix with magnesium prior to casting.

7. The method of making as cast nodular iron from a molten metal comprising 2% to 4.2% carbon by weight comprising the steps of contacting said molten metal with a pretreatment slag comprising 2 parts of a halogen of calcium, 1 part of carbon, and 7 parts calcium mag- 5 nesium carbonate by weight, then inoculating the mix with a nodularizing agent prior to casting.

8. The process of claim 7 wherein the nodularizing agent is magnesium.

References Cited in the file of this patent UNITED STATES PATENTS 2,457,055 Loveless Dec. 21, 1948 2,652,324 Hignett Sept. 15, 1953 2,980,530 Creme Apr. 18, 1961 6 OTHER REFERENCES 

1. THE METHOD MAKING AS CAST NODULAR IRON COMPRISING THE STEPS OF CONTACTING THE MOLTEN METAL WITH A PETREATMENT SLAG COMPRISING 15% TO 21% BY WEIGHT OF HALOGEN OF CALCIUM, ACCOMPANIED BY 3% TO 10% CARBON, AND THE BALANCE CALCIUM MAGNESIUM CARBONATE IN A BASIC OR NEUTRAL VESSEL AND THEN INTRODUCING MAGNESIUM AS A NODULARIZING AGENT PRIOR TO POURING CASTINGS. 